What is graphene:
Graphene is one atomic layer of carbon and the thinnest material known to man. It is a 2 Dimensional sp2 hybridized lattice of carbon atoms arranged in a hexagonal honeycomb configuration. Carbon nanotubes can be formed by rolling graphene along certain axes, and graphite can be formed by stacking graphene vertically.
Graphene has a linear band structure at the Dirac point which gives rise to exotic properties of graphene such as extremely high electron mobility, low resistivity, and broadband absorption. Graphene is ultra-light and has one of the largest specific surface areas among all materials, at 2630 m2 g−. With Young’s modulus of up to 1 , graphene is 200 times stronger than steel, yet flexible and it has an extremely high thermal conductivity of ~ 5300 ⋅ −1⋅−1. Pristine graphene is also impermeable and can act as a perfect barrier which means not even helium can pass through it.
Scientists had theorized about graphene since 1947, but its existence and stability were questioned. Graphene was first isolated and characterized in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester. One Friday, the two scientists removed some flakes from a lump of bulk graphite with sticky Scotch tape. They noticed some flakes were thinner than others. By separating the graphite fragments repeatedly they managed to create flakes which were just one atom thick. They had isolated graphene for the first time. This work resulted in the two winning the Nobel Prize in Physics in 2010 “for groundbreaking experiments regarding the two-dimensional material graphene”.
Properties and Potential Applications
(references: Xinran Wang and Yi Shi, CHAPTER 1: Fabrication Techniques of Graphene Nanostructures, in Nanofabrication and its Application in Renewable Energy, 2014, pp. 1-30 DOI: 10.1039/9781782623380-00001)
Single layer Graphene has many unique electrical, optical, mechanical properties and potential applications.
- High carrier mobility
Electrons in graphene have extremely high electron mobility which makes graphene a prime candidate for electronic applications, including radio frequency and logic transistors. Although the absence of a bandgap places a major obstacle for logic applications, many ways of opening a bandgap have been proposed and tested.
- Conductive and transparent
The resistivity of pristine graphene is expected to be ≈1×10−6 Ωcm, which is lower than the most conductive metal, silver, at room temperature which makes graphene a better electrical conductor than any metals. Optically, graphene is quite transparent over the visible spectra, absorbing 2.3% of the light.
With a combination of high conductivity and transparency, graphene is a prime candidate for transparent conducting films and as a replacement for increasingly costly indium tin oxide (ITO) based films used in touch screens. Another advantage is that graphene is made entirely of carbon and has extremely high mechanical strength and flexibility. Potential near-term applications include touch screen display, e-paper, and organic light-emitting diodes. (OLED)
- Wide-band and tunable optical absorption
Graphene responds to wide-band signals from microwave to ultraviolet and the commonly used fiber-optic communication band which is at 1.55 μm. Combined with the superior carrier mobility, potential applications are ultrafast photodetectors, modulators, terahertz wave detectors and tunable fiber mode-locked lasers.
- Large specific surface area
Since every atom of graphene is on its surface, graphene has one of the largest specific surface areas among all materials, at 2630 m2 g−1. Therefore graphene has great potential in sensor applications with a sensitivity that enables detection of single molecules. Combined with high conductivity and transparency, graphene also has great potential in energy-related applications. For example, supercapacitor electrodes require large conductivity for high specific power and large surface area for high specific energy. Chemically exfoliated graphene is shown to possess both properties and has superior capacitance. Another example is lithium ion batteries. Since (chemically-reduced) graphene has a higher conductivity and can accommodate more strain than many cathode materials, the addition of graphene into these active materials often increased the charging rate and afforded more stable cycling performances.
Some of the other potential applications for single-layer graphene and graphene powders include:
Capacitors are often used as auto components in vehicles. Graphene-based supercapacitors offer much high storage of energy. Supercapacitors are able to hold hundreds of times the amount of electrical charge as standard capacitors and are therefore suitable as a replacement for electrochemical batteries in many industrial and commercial applications.
It is anticipated that graphenes are embedded into plastics such as epoxy to create composite materials that can replace metal in the structure of aircraft, improving fuel efficiency, range and reducing weight. Due to its electrical conductivity, it could even be used to coat aircraft surface material to prevent electrical damage resulting from lightning strikes.
Graphene is able to work on all wavelengths to generate electricity. Thus, graphene-based photovoltaic cells may offer retro-fitted photovoltaic window screens or curtains to help power homes.
Graphene could be used in water filtration systems and desalination systems. Graphene is much stronger and less brittle than aluminum oxide (traditionally used infiltration). Besides, graphene has a high surface area for filtration.
How is graphene made:
The term “ graphene” is sometimes referred to a number of different carbon-based or even graphitic materials and it is a source of confusion. Until recently there were no standardized definitions or terminology for graphene, which created ambiguity as the field and the commercial sector grew. The ISO standard published a few months ago on the vocabulary of graphene clearly states graphene is defined as a “single” layer of carbon which means that a structure with multiple layers of graphene cannot be called graphene, however, the adoption of this standard definition will take time. For example, there is a class of graphene-based materials that we refer to as Graphene powders ( even though graphene powders are sometimes 50 layers of graphene, they are still called “graphene” powders and the community understands that it is not referring to a single layer of graphene )
The excellent properties of graphene that are reported in literature are those of single layer pristine graphene which for the purpose of this document can be categorized to be produced by mainly 2 methods:
Exfoliation: Scotch Tape which was the method through which a single layer of graphene was first isolated and Chemical Vapor Deposition (CVD) method.
Although scotch tape method provides the most pristine and highest quality graphene, it is not scalable, and it is only used in fundamental research projects. CVD Graphene, however, can be commercially scalable. CVD is considered to be the best method for fabricating of high-quality graphene electronics and optoelectronics in large quantities. This method involves depositing gaseous carbon atoms on a copper foil, in a furnace at high temperatures then transferring the single layer graphene film that is grown on copper to any target substrate such as silicon. ( if more information is needed: https://www.graphenea.com/pages/cvd-graphene#.WjXt59-nHic)
CVD graphene is the material that is and will be used in high tech sensors, electronics and photonics and photovoltaic devices.
But there is another class of graphene-based materials called graphene powders which are used in less different sectors such as composites, energy storage, paints, lubricants etc. ( these applications do not require a nanofabrication assembly line or a clean room). Since Graphene powders consist of small stacks of graphene layers (that can even be up to 50 layers of graphene) they do not possess the same excellent properties compared to a single layer graphene, but they can still add significant enhancements if they are added to plastics of composites.
In this class, we could name GNP(graphene nano-platelets), GO ( Graphene Oxide) and RGO ( reduced graphene Oxide). There are many methods to create GNP: for example, it can be produced by microwave-assisted exfoliation of acid-intercalated graphite compound. RGO can be produced by converting graphite to graphite oxide using Hummers’ method followed by sonication exfoliation in water. The exfoliated product is then reduced to graphene using reducing agents.
These materials come in the form of powder (as opposed to a single atomic layer on a substrate) and can be used as additives to make certain structures lighter, stronger, more conductive, more heat conductive or even resilient to corrosion.
For composite materials, graphene powders are dispersed into a matrix to work as reinforcements. Meanwhile, they can also enhance the electrical and thermal conductivities.
Different grades of graphene and graphene powders are used for different purposes. For high-tech electronics and sensor applications, the single layer graphene needs to be quite pristine and free of defects which can be provided by the CDV method, there are currently a few companies in the US that work towards the large-scale production of CVD graphene. When it comes to graphene powders, however, sometimes lower grades of graphene powders or GNP, can provide the added benefits that are required at a more approachable cost. For example, to increase the electrical conductivity of certain plastics you might require a higher grade GNP as opposed to, if you are trying to enhance the mechanical strength of a composite structure and reduce weight, a lower grade graphene powder would be sufficient. Currently, we have a number of graphene powder and GNP companies in the US that provide different grades. However, caution needs to be exercised when it comes to the distinction of graphene powders. The terminology is still used loosely and some unscrupulous companies, sell graphitic materials or carbon black and advertise that as graphene powders or sometimes they even use the word “graphene” in which case, some unsuspecting buyers that don’t have enough information about graphene, buy the product, thinking they will get the amazing properties of “single layer Graphene” they have heard in the news. This emphasizes the importance of industry education through organizations such as National Graphene Association as well as the development of standardized characterization methods for graphene and graphene powders.